1. Field of the Invention
The invention relates generally to the field of semiconductor devices and fabrication and, more particularly, to memory elements and methods for making memory elements.
2. Background of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Microprocessor-controlled integrated circuits are used in a wide variety of applications. Such applications include personal computers, vehicle control systems, telephone networks, and a host of consumer products. As is well known, microprocessors are essentially generic devices that perform specific functions under the control of a software program. This program is stored in one or more memory devices that are coupled to the microprocessor. Not only does the microprocessor access memory devices to retrieve the program instructions, but it also stores and retrieves data created during execution of the program in one or more memory devices.
There are a variety of different memory devices available for use in microprocessor-based systems. The type of memory device chosen for a specific function within a microprocessor-based system depends largely upon what features of the memory are best suited to perform the particular function. For instance, volatile memories, such as dynamic random access memories (DRAMs), must be continually powered in order to retain their contents, but they tend to provide greater storage capability and programming options and cycles than non-volatile memories, such as read only memories (ROMs). While non-volatile memories that permit limited reprogramming exist, such as electrically erasable and programmable “ROMs,” all true random access memories, i.e., those memories capable of 1014 programming cycles are more, are volatile memories. Although one time programmable read only memories and moderately reprogrammable memories serve many useful applications, a true nonvolatile random access memory (NVRAM) would likely be needed to surpass volatile memories in usefulness.
Efforts have been underway to create a commercially viable memory device that is both random access and nonvolatile using structure changing memory elements, as opposed to the charge storage memory elements used in most commercial memory devices. The use of electrically writable and erasable phase change materials, i.e., materials which can be electrically switched between generally amorphous and generally crystalline states or between different resistive states while in crystalline form, in memory applications is known in the art and is disclosed, for example, in U.S. Pat. No. 5,296,716 to Ovshinsky et al. The Ovshinsky patent contains a discussion of the general theory of operation of chalcogenide materials, which are a particular type of structure changing material.
As disclosed in the Ovshinsky patent, such phase change materials can be electrically switched between a first structural state, in which the material is generally amorphous, and a second structural state, in which the material has a generally crystalline local order. The material may also be electrically switched between different detectable states of local order across the entire spectrum between the completely amorphous and the completely crystalline states. In other words, the switching of such materials is not required to take place in a binary fashion between completely amorphous and completely crystalline states. Rather, the material may be switched in incremental steps reflecting changes of local order to provide a “gray scale” represented by a multiplicity of conditions of local order spanning the spectrum from the completely amorphous state to the completely crystalline state.
These memory elements are monolithic, homogeneous, and formed of chalcogenide material typically selected from the group of Te, Se, Sb, Ni, and Ge. This chalcogenide material exhibits different electrical characteristics depending upon its state. For instance, in its amorphous state the material exhibits a higher resistivity than it does in its crystalline state. Such chalcogenide materials may be switched between numerous electrically detectable conditions of varying resistivity in nanosecond time periods with the input of picojoules of energy. The resulting memory element is truly non-volatile. It will maintain the integrity of the information stored by the memory cell without the need for periodic refresh signals, and the data integrity of the information stored by these memory cells is not lost when power is removed from the device. The memory material is also directly overwritable so that the memory cells need not be erased, i.e., set to a specified starting point, in order to change information stored within the memory cells. Finally, the large dynamic range offered by the memory material theoretically provides for the gray scale storage of multiple bits of binary information in a single cell by mimicking the binary encoded information in analog form and, thereby, storing multiple bits of binary encoded information as a single resistance value in a single cell.
Traditionally, the operation of chalcogenide memory cells requires that a region of the chalcogenide memory material, called the “active region,” be subjected to a current pulse to change the crystalline state of the chalcogenide material within the active region. Typically, a current density of between about 105 and 107 amperes/cm2 is needed. To obtain this current density in a commercially viable device having at least one million memory cells, for instance, one theory suggests that the active region of each memory cell should be made as small as possible to minimize the total current drawn by the memory device.
However, such traditional chalcogenide memory cells have evolved into what is referred to as a programmable metallization cell or a plated chalcogenide memory cell for use in a plated chalcogenide random access memory (PCRAM) device. Such a cell includes a chalcogenide material between opposing electrodes. A fast ion conductor material is incorporated into the chalcogenide material. The resistance of such material can be changed between highly resistive and highly conductive states.
To perform a write operation with the memory cell in its normal high resistive state, a voltage potential is applied to a certain one of the electrodes, with the other of the electrode being held at zero voltage or ground. The electrode having the voltage applied to it functions as an anode, while the electrode held at zero or ground functions as a cathode. The nature of the fast ion conductor material is such that it undergoes a chemical and structural change at a certain applied voltage level. Specifically, at some suitable threshold voltage, the metal ions within the chalcogenide material begin to plate on the cathode and progress through the chalcogenide material toward the anode. The process continues until a conductive dendrite or filament extends between the electrodes, effectively interconnecting the top and bottom electrodes to create an electrical short circuit.
Once this occurs, dendrite growth stops, and the dendrite is retained when the voltage potentials are removed. This results in the resistance of the chalcogenide material between the electrodes dropping by a factor of about 1,000. The material can be returned to its highly resistive state by reversing the voltage potential between the anode and cathode to cause the dendrite to disappear. Again, the highly resistive state is maintained when the reverse voltage potential is removed. Accordingly, such a device can, for example, function as a reprogrammable memory cell of non-volatile random access memory circuit.
As mentioned above, the variable resistance material disposed between the electrodes typically is a chalcogenide material having metal ions diffused therein. A specific example is germanium selenide with silver ions. Typically, to provide the silver ions within the germanium selenide material, germanium selenide is deposited onto the first electrode using chemical vapor deposition. A thin layer of silver is then deposited on the glass, for example by physical vapor deposition or another technique. The layer of silver is then irradiated with ultraviolet radiation. The thin nature of the deposited silver allows the energy to pass through the silver to the silver/glass interface to cause the silver to diffuse into the chalcogenide material. The applied energy and overlying silver result in the silver migrating into the glass layer such that a homegenous distribution of silver throughout the layer is ultimately achieved.
Unfortunately, chalcogenide materials are relatively delicate. The nature of the deposition technique used to deposit the silver can damage the chalcogenide material,and, thus, adversely affect the resulting memory cell. Furthermore, it can be challenging to etch and polish chalcogenide materials. Accordingly, it would be desirable to develop memory cell fabrication methods that avoid steps that can damage such materials.